1. Field of the Invention
The present invention relates to a radio frequency power amplifier suitable for power amplification of a radio frequency signal.
2. Description of the Background Art
An amplifier used for a wireless communication device such as a mobile phone or the like generally uses a structure in which a plurality of, for example, two or three, compound semiconductor transistors for radio frequency amplification are connected to each other. As such a compound semiconductor transistor, a hetero-junction bipolar transistor has mainly been used recently for the reason regarding the single positive power source operation or the like. A final-stage amplifier for a mobile phone having an output power of about 1 to 3 W is provided by connecting a plurality of transistors in parallel to combine the outputs from the transistors, in order to obtain radio frequency characteristics and a high output. FIG. 10 shows an example of a conventional radio frequency power amplifier 100 having such a structure. See, for example, U.S. Pat. Nos. 5,321,279 and 5,608,353.
With the conventional radio frequency power amplifier 100 shown in FIG. 10, a direct current bias voltage (DC) given from a bias circuit (Bias) is supplied to bases (B) of transistors TR101 through TR10n via corresponding resistors Ra101 through Ra10n. A radio frequency signal (RF), which is an AC signal, is input to the bases of the transistors TR101 through TR10n via corresponding capacitors C101 through C10n. The reason why the bias voltage (DC) and the radio frequency signal (RF) are input to the bases of the transistors TR101 through TR10n via separate paths is as follows.
The transistors TR101 through TR10n generate heat when performing a high output operation because the current density of the AC current is raised. The heat generation is not uniform among all the transistors TR101 through TR10n due to a characteristic dispersion among the transistors TR101 through TR10n or the like. A specific transistor having a high temperature may cause thermal runaway due to excessive heat generation during operation and destroy the device due to an increase in the base current. According to a conceivable technique for suppressing the thermal runaway, when base voltages of the transistors TR101 through TR10n increase, the resistance values of the resistors Ra101 through Ra10n are increased such that the base bias current supplied from the bias circuit (Bias) decreases.
In the above-described radio frequency power amplifier 100, the thermal runaway of the transistors Ra101 through Ra10n is suppressed and thus uniform operation thereof is realized by increasing the resistance values of the resistors Ra101 through Ra10n. 
However, the resistance values of the resistors Ra101 through Ra10n cannot be very large because excessively large resistance values of the resistors Ra101 through Ra10n decrease a power gain of a radio frequency signal. Namely, the operation uniformity among the transistors TR101 through TR10n improved by increasing the resistance values of the resistors Ra101 through Ra10n (improvement in the destruction resistance) contradicts the radio frequency power gain improved by decreasing the resistance values of the resistors Ra101 through Ra10n (improvement in the radio frequency characteristics). It is very difficult to improve both.